When data communication is conducted, it is necessary to verify that a network device such as a receiving device and a transmitting device operate normally. Conventionally, a verifier conducts such verification by connecting a device to be verified to a verifying apparatus corresponding to the device through the same cable. In the case of a transmitting device, verification is conducted by connecting a transmission terminal of the transmitting device to a reception terminal of a verifying apparatus for the transmitting device on a one-to-one basis. In the case of a receiving device, verification is conducted by connecting a reception terminal of the receiving device to a transmission terminal of a verifying apparatus for the receiving device on a one-to-one basis. In the case of a transmitting and receiving device having both transmission and reception functions, however, a verifying apparatus for a transmitting device and a verifying apparatus for a receiving device cannot be connected to the transmitting and receiving device at the same time to conduct verification. Therefore, the transmission and reception functions of the transmitting and receiving device are separately verified by using separate verifying apparatuses. For example, a verifier first verifies the transmitting function by using a verifying apparatus for a transmitting device and then verifies the receiving function by using a verifying apparatus for a receiving device.
Japanese Laid-Open Patent Publication No. 5-102891 (Patent document 1) discloses a testing system for testing data communication by a self-testing method. A verifying apparatus described therein includes a test data transmitting section for transmitting test data, a test data receiving section for receiving test data, a first signal switching section for folding back a signal, a second signal switching section for supplying and reading test data, a comparing section for comparing a value of transmitted test data with a value of received test data, and an apparatus status control section for controlling each section and having a self fault diagnosis function. When this verifying apparatus is set to a test mode by the apparatus status control section, the first signal switching section switches to a signal fold-back position. The test data transmitting section then transmits a test signal to a circuit to be tested through the second signal switching section. An output signal from the circuit is folded back in the first signal switching section and transmitted back to the circuit. The signal thus transmitted back to the circuit is transmitted to the test data receiving section through the second signal switching section. The test data comparing section compares the test data in the test data transmitting section with the test data in the test data receiving section and determines if data communication is done normally or not.
With recent prevalence of a next-generation multimedia interface, AV (Audio Visual) devices having this interface to transmit information have been developed one after another. In communication using this interface, prescribed data according to a prescribed standard (e.g., an encrypted video signal and an encrypted audio signal) is transmitted together with prescribed information to be used to process the prescribed data (e.g., an encryption key). An example of such an interface is an HDMI (High Definition Multimedia Interface). An HDMI is formed by a high-speed interface (TMDS: Transition Minimized Differential Signaling) and a low-speed interface (DDC: Display Data Channel). Transmission of baseband video and audio data is conducted in the high-speed interface, and transmission and reception of authentication information that is used for authentication between devices, exchange of encryption keys that are required for encryption and decryption of data, and transmission and reception of device information for notifying the other device (a device to which the device is connected) a device status of the device are conducted in the low-speed interface.    Patent document 1: Japanese Laid-Open Patent Publication No. 5-102891    Patent document 2: U.S. Pat. No. 6,940,263    Patent document 3: U.S. Pat. No. 6,385,236    Patent document 4: U.S. Pat. No. 5,953,372